Thermoforming and Casting Flashcards
Thermoforming Process: Heating
Usually by radiant electric heaters, located on one or both sides of starting plastic sheet at roughly 125mm
Thermoforming Overview
- Relies on softening thermoplastic above Tg for amorphous polymers or slightly below Tm for semicrystalline
- Common plastics: PS, cellulose acetate and CAB, ABS, PVS, PMMA, PE and PP
- Examples: food packaging (mostly film), advertising signs. refridgerator liners, manufacturing collation trays etc.
Thermoforming Process
- Clamp a sheet of material
- Heat to the sag point
- Force against the surface of a mould by the use of a vauum, or air pressure, or both
- Cool and remove
Methods of Thermoforming
- Vacuum forming
- Pressure forming
- Mechanical forming
- Free blowing
- Drape forming
- Plug-assisted forming
Vacuum Forming Process
- Sheet clamped in frame and heated
- When sheet is rubbery, heat is removed and it’s placed in a concave mould cavity
- Vacuum draws the sheet into the cavity
- Plastic hardens on contact with cold mould surface - removed then trimmed
Pressure Forming Process
- Sheet is placed over a mould cavity
- Positive pressure forces the sheet into the cavity
Mechanical Forming Process
- Clamp heated rubbery sheet between two mould halves
- Once mould is closed, vacuum is applied to the female half of the mould
Positive and Negative Moulds
- A positive (or male) mould has a convex shape
- A negative (or female) has a concave cavity
- Each produces a different pattern of thinning in a given part
Free Blowing
- Part-spherical shapes can be produced by simple blow forming of heated sheets of thermoplastics
- Can also be blown into shaped female moulds
Thermoforming Advantages
(4)
- Inexpensive tooling (often Al)
- Vacuum mould requires holes placed in appropriate positions - size less than 0.5mm to avoid leaving marks
- Medium production rates
- Scrap is 15-50% (called skeletal) - recycled to sheet extruders or for secondary use
Thermoforming Disadvantages
(3)
- High part cost relative to injection moulded parts
- Non-uniform gauge due to film stretching
- Process is limited by geometry
Casting Thermoplastics
- Monomer, catalyst, activators are mixed and heated above m. pt (part formed after polymerisation)
- Materials include: acrylics, polystyrene, polyamides (nylons) and vinyl (PVC)
Casting Thermosets
- Mixed and poured
- Materials include: epoxies, phenolics, ployurethanes, polyesters
Casting Products
Gears (nylon), bearings, wheels, thick sheets, lenses, components requiring resistance to war
Casting Processes
- Monomers casting
- Potting and encapsulation
- Mould is simpler (less costly)
- Cast item is free of residual stress and viscoelastic memory
- Suited to low production quantities
Monomer Casting
- Uses controlled chemical reaction of polymer materials
- Monomer is mixed with a catalyst, additives etc, before pouring - mixture then polymerises
- Moulds can be epoxy, plaster, wood, sheet metal or even rubber
- Produces high molecular mass polymer sheet for certain thermoforming applications (e.g. aircraft windows)
Monomer Cating Advantage
Requires minimum capital expenditure
Monomer Casting Disadvantage
Difficult to automate
Potting and Encapsulation
- Processes for electrical assemblies where the surrounding serves as a dielectric
- Involves casting liquid resin (e.g. epoxies, urethanes and silicones) around an electrical component (e.g. transformer) to embed it in the plastic
- Used in applications where components need protection (e.g. physical and thermal shock, vibration, moisture, corrosion)
Applications of Potting and Encapsulation
Electronic and electrical components, sensor assemblies, power supplies, PCBs, transformers
Potting
Comonent is covered in resin to guard it against potential environmental threats or to insulate electrically using a “pot” housing or case, which becomes part of the component
Encapsulation
- Involves placing the assembly or component within a machined cavity of a mould tool
- Then tool is then filled with casting resin until set
- Component and hardened resin are then removed from the mould to be placed in an assemble
Advantages of Potting and Encapsulation
(7)
- Non-corrosive
- Resistes moisture, vibration and T extremes
- Enhances electrical insulation
- Flame retardent resins available
- Improves assembly robustness
- Design in strain relief
- Custom designs/shapes and logos possible
Disadvantage of Potting and Encapsulation
Heat from the curing reaction may cause damage to components
